[Purdue University] Sixty-six million years ago, an asteroid about the size of Mount Everest smacked into Earth near where the Yucatan Peninsula is today. That impact created the Chicxulub crater and may have knocked out about three-quarters of all living species — including most of the dinosaurs.
Planetary scientists from Purdue University have discovered the key to how one rock — albeit a big one — could have devastated an entire global ecosystem: The asteroid sent up a cloud of planet-smothering dust, bathing the Earth in thermal radiation so intense that most species simply could not survive.
The study, published in the Journal of Geophysical Research: Biogeosciences, found that the impact’s dust cloud so effectively trapped heat that almost none could escape into space. It superheated the upper atmosphere, caused widespread wildfires and had massive effects on any life form that couldn’t take shelter from it — underground, underwater or using some other strategy.
Brandon Johnson, the study’s lead author, is an expert in craters and impacts. He studies how planetary bodies impact — literally — other planetary bodies, including the Chicxulub impact that many scientists believe triggered the extinction of the dinosaurs along with much of the planet’s other life.
A professor in the Department of Earth, Atmospheric, and Planetary Sciences at Purdue, Johnson studies the physics of planetary collisions to pin down exactly what happened and how: a forensic crash investigator 66 million years after the fact.
“That much kinetic energy has to go somewhere and eventually it is converted to heat,” he said. “With the dust cloud trapping thermal radiation, it was like the surface and everything on it was being charbroiled. And that’s what killed the dinosaurs and all the other animals. It’s not the blast wave from the impact or the fireball — those don’t go very far. It’s the global ejection of this vapor plume material that makes this a global extinction event. Without the dust cloud, it would have been bad. It would have killed a lot of creatures. But it wouldn’t have been a planetary catastrophe.”
The sky is falling
In previous research with Jay Melosh, Johnson’s advisor and former collaborator at Purdue, Johnson discovered that when the Chicxulub asteroid hit, it vaporized over 1,000 cubic kilometers of material — largely rock and dirt, though also whatever animals and plants were in the way.
All that material expanded in a plume above the atmosphere. As the vapor rose into the air, it cooled and then condensed, just as water vapor does. But instead of condensing into raindrops, the impact vapor condensed into droplets of rock, dubbed spherules. The spherules are relatively large as tiny specks go — about 250 micrometers in diameter, or about half the size of a grain of sugar.
As the spherules fell back to Earth, the resistance of the surrounding air heated them up and turned them into miniature versions of the asteroid impact itself. That additional heat added to what was generated in the initial impact.
Paleontologists have uncovered spherules from all over the world, including from the gills of paddlefish at a site in North Dakota who likely died on the day the asteroid hit. Previous research had found that the spherules were made up of material of terrestrial origin — nothing from the asteroid itself. That material, it turns out, did not precipitate into spherules but instead became a cloud of fine dust that ballooned across the entire planet. That asteroid dust later insulated the planet like a smothering blanket that wouldn’t allow heat to escape.
The dust cloud changed the impact from merely tragic to apocalyptic.
“With this fine dust cloud capping the atmosphere, the heat radiation can’t escape to space,” Johnson said. “So the only place that radiation can go is back down to the earth. Without fine dust, this still wouldn’t have been a good day for dinosaurs. It would have killed some off, but it probably wasn’t enough to start many wildfires. With the fine dust, the Cretaceous animals received 17 times the dose of thermal radiation that is 100% lethal to humans. It was enough to ignite grass, pine needles, lichen and maybe even directly ignite wood.”
With the blanketing clouds and boiling heat, Johnson’s description sounds like the surface of Venus. But the comparison, he says, is not quite apt.
“It probably looked more like hell than like Venus,” he said. “The clouds would have blocked daylight so to any animal, like humans, that don’t see far into the infrared, the surface would have probably looked dark. You would only have been able to see a reddish glow from the wildfires. But there really wouldn’t be anything alive to do the seeing anyway, except for any animal that was able to burrow or swim or shelter in some way, to find a way to survive. It would have been terrible.”
Planetary scientists Brandon Johnson and Alexandria Johnson, experts in craters and clouds respectively, analyzed the physics of the Chicxulub impact to understand how one rock, even a big one, could have devastated an entire global ecosystem: The asteroid sent up a cloud of planet-smothering dust, bathing Earth in thermal radiation so intense that most species simply could not survive. (Purdue University photo/Kelsey Lefever)
Ashes to ashes, dust to dust
To analyze how well the dust cloud would have insulated the planet, the team brought in Alexandria Johnson, an expert on clouds.
A Purdue assistant professor of earth, atmospheric and planetary sciences, Alexandria Johnson studies clouds both on Earth and on other planetary bodies. She looked at the physical properties of the individual dust particles and the dust cloud as a whole to determine the cloud’s radiative properties: how it handles heat.
“We found that the cloud layer was so impermeable that it trapped almost all of the heat from the falling spherules near the surface of the planet — the dust is essentially acting like a lid on a pot,” she said. “It is because of this dust layer that anything that couldn’t shelter itself somehow — underground or underwater — would have gotten baked or fried. Burrowing underground protected those animals from the heat because there are different thermodynamics between the atmosphere and the solid surface.
The dust layer would have had ongoing impacts beyond the fatal heat. The dust could have taken years, or even decades, to settle, and the particles themselves could have represented an ongoing health threat to organisms.
While the spherules, the droplets of rock precipitated from the impact, were visible to the naked (human) eye at 250 micrometers in diameter, the dust cloud was made up of particles 10 times smaller: 2.5 micrometers in diameter — 30 times smaller than a human hair.
“I study the clouds and air pollutants in our skies today,” Alexandria Johnson said. “The 2.5-micrometer dust we studied here is the same size as the smoke particles we worry about with wildfires. It gets in people’s lungs and even in our bloodstreams, which is why it’s such concern for public human health. So even if critters were to survive the charbroiling, they would likely have had lasting health effects from these particulates as well. But when you look at the time scales of the cooking versus the inhalation risks, the cooking is much faster.”
Heat and Wildfires During the K-Pg Mass Extinction Enhanced by Fine Dust, JGR Biogeosciences (open access)
Astrobiology,
